#include <iostream>
#include <cstring>
#include <algorithm>
#include <cstdio>
#include <cmath>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <set>
#include <bitset>
#include <utility>
using namespace std;

#define mm(a, n) memset(a, n, sizeof a)
#define mk(a, b) make_pair(a, b)

const double eps = 1e-6;
const int INF = 0x3f3f3f3f;

typedef long long LL;
typedef unsigned long long ULL;
typedef pair<int, int> PII;
typedef pair<double, double> PDD;
typedef pair<LL, LL> PLL;
typedef pair<int, LL> PIL;

inline void quickread() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
}

int n, m;
LL res;
priority_queue <PII,vector<PII>,greater<PII>> pq;

inline void solution() {
    cin >> n >> m;
    while (m -- ) {
        int c, p;
        cin >> p >> c;
        pq.push(mk(p,c));
    }
    while (n > 0) {
        if (n >= pq.top().second) {
            n -= pq.top().second;
            res += pq.top().second * pq.top().first;
            pq.pop();
        } else if (n < pq.top().second) {
            res += n * pq.top().first;
            n = 0;
        }
    }
    cout << res << endl;
}

int main() {
    // freopen("input.txt", "r", stdin);
    quickread();
    solution();
    return 0;
}